Diffusion cladding aluminum article with a diffused zinc coat



United States Patent 3,390,970 DIFFUSION CLADDING ALUMINUM ARTICLE WITH A DIFFUSED ZINC COAT Marvin R. Bothwell, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Original application Aug. 20, 1962, Ser. No. 218,128, now Patent No. 3,268,358, dated Aug. 23, 1966. Divided and this application Oct. 21, 1965, Ser. No. 509,465

1 Claim. (Cl. 29-197) ABSTRACT OF THE DISCLOSURE The present invention relates to an aluminous base metal article wherein the surface metal is preferentially sacrificial to the core metal, exhibiting a gradient of diffused zinc from the surface into the base metal.

This is a division of application Ser. No. 218,128 filed Aug. 20, 1962, now Patent 3,268,358.

Under present art aluminous base metals are metallurgically and laminarly clad with other aluminous metals containing various additive metals such as silver, cadmium, platinum, barium, bismuth, tin, and zinc sufficient in concentration to impart to the cladding layer a more active potential than that of the core metal so as to direct sacrificial corrosive attack to the cladding in preference to the core. In practice however, metallurgical and laminar cladding are economically or technically limited to sheet or plate core metal, and are comparatively expensive when applied to tubular goods and cannot be applied to irregular configurations such as castings.

In view of the limitation in metallurgical and laminar cladding techniques there is needed in the art for a simple and economical process whereby aluminous base metals of substantially any irregular configuration, the surfaces of which, can be processed to obtain a sacrificial nonlaminar cladding of a higher electrode potential than that of the base metal which will, therefore, corrode in pref erence to the base when in contact with a corroding medium and prevent pitting and other type corrosion of the base metal.

Further reference in this specification to aluminous base metals pertains to metals containing predominantly aluminum. The term thus includes elemental aluminum and the aluminum base alloys. Also, reference in this specification to zinc for initial coating purposes refers to metals consisting predominantly of zinc, and also refers to compounds containing zinc which are reducible to zinc metal by the aluminum during the diffusion heat treatment.

-It is the object of this invention to provide a method whereby zinc is diffused into the surface of an aluminous metal base rendering a greater electrode potential to said aluminum-zinc surface gradient so that any corrosive attack will be preferentially directed to the diffused zinc enriched surface rather than to the base.

It is also the object of this invention to provide an economical process whereby zinc can be diffused into the surface of an aluminous base metal of any configuration.

A further object is to provide a process whereby the zinc may be diffused into the surface of the base metal with little or no impairment of the physical properties of the base article. A related object is to provide aluminous base metal articles having a protective diffused zinc surface the electrode potential of which is greater than that of the aluminous base metal.

This invention is based upon the discovery that the surface of an aluminous base metal can be given a substantially greater electrode potential than the base by depositing on the surface thereof a coating consisting essentially of zinc metal and diffusing such coating into the surface of the base by heat treating the coated article, thereby providing a zinc enriched surface and composition gradient such that when placed in a corroding atmosphere cathodic protection results in that galvanic attack is directed to the diffused zinc enriched surface in preference to the aluminous base. Diffusion cladding under the invention in this manner is accomplished without serious impairment of the physical properties of the base article.

In practicing the invention any aluminous base metal configuration is coated with zinc metal or alloy thereof by any one of various methods of deposition known to the art so as to provide an initial zinc coating of about 0.10 Ing /cm. to about 10.0 mg./cm. The coating is then diffused into the surface of the base by heat treating at a temperature within the range from about 500 F. to about 1100 F. for about 0.1 to about hours. In any case, the heat treatment is to be sufiicient to accomplish at least a 0.001 inch penetration having at least a 0.5 percent zinc concentration at said depth penetration. Preferably, the invention is practiced with the deposited coating of zinc to be diffused being approximately 0.8 mg./cm. with heat treatment to effect diffusion being carried out at about 970 F. for about 5 hours.

The application of heat treatment time and temperature to effect diffusion directly depends upon the initial zinc concentration per unit area of base metal surface, that is, an initial concentration in the lower limits of the aforementioned coating range of 0.10 mg./cm. to 10.0 mg/cm. will require a lower heat treatment temperature and time to achieve optimum results, whereas, a heavy initial coating within said range will require a greater temperature and time. The time and temperature combination in relation to one another however, is inverse, that is, heat treating at higher temperatures will require less time to effect proper diffusion of a given initial zinc coating than with lower temperatures.

One method of deposition found to be particularly satisfactory to obtain the initial zinc coating for diffusion is a two step zinc immersion process whereby the aluminous base metal is first activated in a dilute aqueous NaOH solution and then immersion plated with zinc by simple displacement in an alkaline zincate solution. Other methods of deposition known to the art are also applicable to the invention to obtain the initial zinc coating, such as metal spraying, electrodeposition, and mechanical plating with metal powder using abrasives.

Table I indicates preferable, but not limiting, combinations of initial zinc coating weights to heat treatment temperatures and time.

This invention provides a simple and economically practical process whereby all or any desired part of the exposed surfaces of any configuration of aluminous base metal articles can be given an electrode potential greater than that of the underlying base by enriching said surface by diffusion of zinc to provide a sacrifical zinc diffused surface which in the presence of a corroding medium will preferentially bear corrosive attack. In addition, the process causes, and the product shows, little or no impairment of the physical properties of the core metal.

I claim:

1. An aluminous base metal article wherein the surface metal is preferentially sacrificial to the core metal, exhibiting ditfusion from the surface into the base metal of about 0.10 mg. to about 10.0 mg. of zinc per square centimeter of surface as a gradient in zinc concentration from said surface into the base metal such that the zinc concentration at 1 mil below the surface is at least 0.5 Weight percent. V I

References Cited UNITED STATES PATENTS 8/1966 Bothwell 117-131 HYLAND BIZ-OT, Primar Examiner. 

